Author(s): Ana Mendonca; C. J. E. M. Fortes; F. S. B. F. Oliveira; M. G. Neves; Pedro Bicudo; F. Sancho; Rui Reis; Juana Simao; Cesar Costa

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Keywords: 3D experiments; COULWAVE; Coastal structure; Breaking

Abstract: The National Laboratory of Civil Engineering (Portugal) is undertaking a characterization and feasibility study of a multifunctional submerged detached breakwater (MSDB) in front of Praia da Vagueira, in the central West coast of Portugal, for the Portuguese Environment Agency, I.P. (APA). The study zone includes a 620 m long seawall, built in 1978 and recently raised, and a groyne located at the southern extreme of the defence. It is intended that the MSDB is multifunctional, contributing, by order of priority, to: a) reduce the risk of coastal overtopping in the Praia da Vagueira urban agglomeration; b) promote the accumulation of sand on the emerged beach (salient growth), in front of the seawall, reinforcing the natural defense against erosion and coastal flooding, and improving the beach touristic use; and c) create conditions to promote surfing waves, boosting the economy associated with this activity. A physical and numerical modeling study was developed to investigate the last objective, i.e., the influence of the MSDB configuration and the local wave conditions in the surfability parameters. The hydrodynamic analysis is based on the parameters: surf line, Iribarren number, peel angle and wave height amplification. The Boussinesq-type COULWAVE model is used to study the wave hydrodynamics in the vicinity of the MSDB. The model was used for regular waves, forced by the wave height, period and direction parameters at the offshore boundary. Numerical tests were performed without and with the MSDB for wave conditions representative of the summer and winter maritime seasons. In the study area, the values of the significant wave height, the horizontal velocity near mid depth of the water column and at the free surface, the wave direction and the position of the breaking line were obtained. With these results, the surfability parameters which characterize the functioning of the MSDB in surfing were determined. Directions in the domain (which is not a direct result of COULWAVE model) were computed with DIWASP tool and the Direct Fourier transform method. 3D laboratory experiments were performed at one of LNEC’s wave basins at the reduced scale 1:65. Tests without and with the MSDB were performed in order to evaluate the wave breaking conditions. The results, from physical and numerical tests, were analyzed and compared in terms of significant wave height around the MSDB, breaking line location and length, Iribarren number and peel angle. The complementarity of the results improves the analysis on the ability of this structure to promote surf waves, one of this main goal.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221008

Year: 2022